The Epidermal Growth Factor Receptor (EGFR), a cell surface membrane signaling protein, has emerged as one of the most promising molecular targets for anti-cancer therapy. The long-term objective of this research proposal is to gain new information to guide the optimal use of molecular inhibitors of EGFR in cancer patients. The proposed studies will take advantage of newly established EGFR inhibitor-resistant human tumor cells to examine cellular interactions between EGFR inhibitors and radiation, and explore strategies to overcome intrinsic or acquired resistance to EGFR inhibitor therapy. Promising leads from preclinical findings in tumor model systems will then be further examined by molecular analysis of tumor specimens from patients enrolled in a clinical trial. The first scientific aim is to characterize specific differences between EGFR inhibitor-resistant and parental tumor cells with regard to their molecular footprint and radiation response profile. These in vitro and in vivo studies will examine molecular distinctions between EGFR inhibitor-resistant and sensitive tumors, and identify gene and protein expression differences using high-throughput analyses. The second scientific aim is to explore the functional significance of altered protein and gene expression changes with regard to subsequent EGFR inhibitor and radiation response. These preclinical studies will include systematic evaluation of promising molecular targets that may play a role in the resistance mechanism. The third scientific aim is to investigate these molecular targets that characterize the EGFR inhibitor-resistant profile, and correlate their expression with ultimate clinical outcome in human tumor specimens from patients enrolled in a national cancer trial. It is anticipated that these studies will provide valuable information regarding mechanisms of response and resistance to EGFR inhibitors, and thereby facilitate the design of innovative cancer treatment strategies to improve outcome. Public Health Description: Many of our most promising cancer drugs show beneficial impact in only a minority of patients with advanced tumors. The majority of tumors exhibit some form of resistance to drug treatment. In this research proposal, we seek to increase the percentage of cancer patients who benefit from leading cancer drugs through the identification and disabling of specific tumor cell molecules that promote treatment resistance.